1. Field of the Invention
The present invention relates to an electrostatic latent image measuring device, an electrostatic latent image measuring method, and an image forming device, which measure a surface potential distribution and a surface charge distribution of a photoconductor and analyze the surface.
2. Description of the Related Art
An electrophotographic image forming device such as a copying machine or a printer uses a photoconductor.
The following processes are performed relative to the photoconductor.                A charging process which uniformly charges the electrophotographic photoconductor.        An exposing process irradiates light corresponding to an image onto the uniformly charged photoconductor, removes the charge in the portion onto which the light is irradiated, and forms an electrostatic latent image.        A developing process, which forms a visible image by toner on the electrostatic latent image by transferring charged fine particles (hereinafter, referred to as toner) onto the above charged portion.        A process which transfers the visualized (developed) toner image on paper or another transfer member.        A process which fuses the toner forming a transfer image on paper or a transfer member.        A cleaning process which cleans the residual toner on the photoconductor after transferring the toner image on paper or a transfer member.        A process which eliminates an electric charge remaining on the photoconductor.        
The photoconductor is the heart of the image forming device. By analyzing the influences from the above processes to the photoconductor, an essential problem for forming an image can be discovered.
Moreover, solving the essential problem may lead to a new invention or new discovery.
Since a process factor and a process quality in the above processes significantly affect a quality of an output image, it is required to directly observe the photoconductor in some way for evaluating the processes.
Thereby, the quality of the final output image can be directly analyzed. It is also expected that a significant response or suggestion can be obtained regarding an improvement in the quality of the processes.
More particularly, the above processes are performed to the photoconductor, and a method of analyzing these processes is introduced. Thereby, the formation of the latent image on the surface of the photoconductor can be analyzed, and it is expected to obtain extremely important information for obtaining a high quality image by directly or indirectly evaluating the quality of the electrostatic latent image after exposing.
By understanding a mechanism in which exposure light is converted into an electrostatic latent image, the information to be obtained from the electrostatic latent image can be used for designing and developing an optical system for exposing. It is expected that an image forming device which can form a high quality image at low cost, for example, can be designed.
However, it is extremely difficult to measure an electrostatic latent image, and an actual image forming device can not measure the electrostatic latent image at present.
For example, as a method of measuring an electrostatic latent image, an SPM (scanning probe microscope) is introduced. In such a device, a head sensor such as a cantilever is moved closer to a sample having an electric potential distribution, and the electrostatic attractive force and the dielectric current generated by the mutual influence between the electrostatic latent image and the cantilever are measured, and then these are converted into the electric potential distribution. However, a method of measuring an electrostatic latent image which visually displays the results of the measurement has not been obtained yet.
As a dielectric current type device, inventions described in JP3009179B and JP H11-184188A are known.
However, in these devices, it is necessary to move the head sensor closer to the sample. In order to obtain a spatial resolution of 10 μm, for example, it is necessary to set the distance between the sensor and the sample to 10 μm or below.
Accurate distance measurement is required for setting such a condition, and the measurement needs to be performed many times.
In the measurement method performed many times, an actual electrostatic latent image can not be measured because natural discharge or absorption of a substance occurs during the measurement, and the condition of the latent image varies from hour to hour during the measurement by the influence of the sensor itself, so that the real-time condition of the electrostatic latent image can not be obtained.
As a more realistic measurement method, a method of applying an electric charge to coloring toner, visualizing (developing) an electrostatic latent image by absorbing the charged coloring toner onto the electrostatic latent image with a coulomb force, and transferring the toner image on paper or a tape, can be adopted.
However, in these methods, since the development and transfer processes are conducted, the electrostatic latent image is not directly measured.
To the present inventor's knowledge, a method which directly picks up various events on the surface of the photoconductor has not been developed yet.
On the other hand, a method of measuring an electric potential pattern using an electron beam is conventionally known.
This method is used for analyzing failure of an LSI. In this method, it is necessary for the sample to be a conductor.
However, considering a conductive property of a semiconductor such as Si, the material of the photoconductor is rather an insulating material, so that such a measuring method can not be applied to measure a photoconductor sample.
In the conductive sample, the electric potential distribution can be maintained for a long period of time by applying a constant current, a low voltage of about 5V is applied, and also the range is narrow. Therefore, a charge-up phenomenon does not occur.
Accordingly, if an electric beam is irradiated onto such a conductive sample, a problem regarding the measurement does not occur in the photoconductor in which the above electric potential changes.
For this reason, this method can not be directly applied to a general photoconductor.
In a general dielectric body, an electric charge can be semipermanently maintained. However, in the photoconductor, the electric charge can not be maintained for a long period of time because the photoconductor has some conductive property, so that the surface potential of the photoconductor is lowered by dark decay with time.
Since a time of which the photoconductor maintains an electric charge is about several tens of seconds at most even in a dark room, when trying to observe an electrostatic latent image in a scanning electron microscope after charging and exposing, the electrostatic latent image formed on the photoconductor sample disappears in the preparation step, so that the electrostatic latent image can not be observed.
The photoconductor for use in an electrophotographic process generally has a cylindrical shape.
It is desired to measure the distribution of the electrostatic latent image on the cylindrical photoconductor at high resolution without destroying the distribution.
It is also desired to provide a new technique such as a device which evaluates the change in the electrostatic latent image of the photoconductor by the time degradation associated with the use of a unit for generating an electric charge distribution on the photoconductor, a device which forms an electrostatic latent image, a measuring method and the like.
In addition, the applicants of the present application have already filed an invention regarding the above new technique (reference to JP2006-294515A, for example).
It is known that the electric charge is spatially scattered in the sample.
For this reason, the surface charge described herein means a condition in which the electric charge distribution is large in the in-plane direction compared to that in the thickness direction.
The concept of the electric charge includes not only an electron but also an ion.
The surface charge also means a condition in which the surface includes a conductive portion, and an electric potential distribution is generated on the surface of the sample or the vicinity thereof by applying a voltage to the conductive portion.
Conventionally, as a method which measures a surface potential of a photoconductor or the like, there is a method which moves a sensor head closer to a sample having electric potential distribution, measures an electrostatic attractive force and an induction current resulting from the mutual influences, and converts the electrostatic attractive force and the induction current into an electric potential distribution.
In this method, the resolution is low at about several millimeters, and 1 micron resolution can not be obtained.
A method which measures an electric potential in 1 micron order by means of an electron beam is conventionally known as a method which evaluates an LSI chip.
However, this evaluation is conducted on a conductive portion in the LSI in which a current flows, the electric potential is low at about +5V, and also an electric potential is limited. Therefore, this evaluation can not be conducted on a negative electric charge of several hundred to several thousand V, which is a target to be measured in the present invention.
A method which observes an electrostatic latent image by means of an electron beam is described in JP H03-049143A, for example.
In the conventional observation method, a sample is limited to an LSI chip or a sample capable of storing and maintaining an electrostatic latent image.
Namely, an electrostatic latent image formed on a general photoconductor in which dark decay occurs can not be measured.
Since a general dielectric body semi-permanently maintains an electric charge, even if time-consuming measurement is conducted after forming an electric charge distribution, the result of the measurement is not affected.
However, since the resistance value of the photoconductor is not infinity, the electric charge can not be maintained for a long period of time in the photoconductor. For this reason, the dark decay occurs, and then the surface potential is decreased with time.
A time of which the photoconductor can maintain the electric charge is several tens of seconds at most.
Consequently, when trying to observe an electrostatic latent image in a scanning electron microscope (SEM) after charging and exposing, the electrostatic latent image is disappeared in the preparation stage.
In the invention described in JP H03-200100A, not only is a wavelength used different, but also a latent image of a beam profile, a desired beam diameter and no line pattern can not be formed.
As a result, the present inventors invented a method which measures an electrostatic latent image even on a photoconductor sample having dark decay (refer to, for example, JP H03-200100A and JP 2003-295696A).
If the surface of the sample includes an electric charge distribution, an electric field distribution according to the electric charge distribution on the surface is formed in a space.
The secondary electron generated by the incident electrons is thereby brought back by the electric field, and the number of electrons which reach a detector is reduced.
Therefore, a contrast image according to the electric charge distribution on the surface, in which a portion having a strong electric field is dark and a portion having a weak electric field is bright, can be detected.
When exposing, the exposed portion becomes black and the non-exposed portion becomes white, and the electrostatic latent image formed by the exposure can be measured.
In the meanwhile, there is a method which forms an electrostatic latent image by turning on and turning off a semiconductor laser having a wavelength from a visible light area to an infrared light area (hereinafter, referred to as a LD (laser diode)) as an exposure light source for forming an electrostatic latent image.
The semiconductor laser oscillates a laser by applying a reference driving current or more, and a constant reference driving current or below (bias current) is always applied to the semiconductor laser even during the time of turning-off.
If the bias current is supplied to the semiconductor laser, the semiconductor laser emits light by an emission mechanism similar to that of an LED.
This means that the semiconductor laser emits light even in the turning-off condition when the bias current is supplied.
In this case, the light volume is weak, so this light volume does not affect the electrostatic latent image when the irradiation time is short.
However, if the irradiation time is long, the integrated light volume is increased. If the integrated light volume reaches a required exposure amount, the electrostatic latent image is formed.
As a result, a desired electrostatic latent image can not be formed.
For this reason, in order to form a desired electrostatic latent image, it is necessary to control the irradiation time of light to be irradiated on the sample by the emission with the bias current in the turning-off period.
As another problem, there is a problem that the scanning area is changed if the sample is charged.
By the electric charge on the surface of the sample, the electric field in the device is changed, and the orbit of the scanning electron is curved.
For this reason, if data is loaded as an image with a condition which is the same as the condition in the non-charging, the size of the image is slightly different from the size of the actual image.
Accordingly, it is required to accurately measure the coordinate of the sample from the loaded data.
It is difficult to previously estimate the amount of change in the scanning area because various conditions such as a charging electric potential and a height of a sample are changed.
A conventional method of measuring an electrostatic latent image includes a method of correcting image data according to a reference sample in which a size of a hole and a projection are previously known.
However, since the standard material is generally a conductive sample, the standard sample can not be charged.
An insulated sample can be charged, but a desired charging electric potential can not be applied to a desired area which becomes a standard, and the electric charge can not be removed in the insulated sample.
A sample from which an electric charge can be removed includes a photoconductor sample. However, the photoconductor sample is easily affected by electrostatic fatigue and light fatigue, so that the charging condition is changed, and the photoconductor sample can not be used as the standard sample.
A method which provides a projection on a sample or damages a sample for use as a standard sample is not appropriate because it damages the sample (refer to, for example, JP2004-251800A and JP2008-233376A).